Thread level prefixes are used in the "physics" forums and a few others. They allow thread starters to better express their own sophistication and that of the content they are posting. It helps other users equally understand the level of the poster and content. It will also allow users the ability to scan and filter for the thread level they are interested in.
On the forum list you can see the level prefixes to the left of the title. B = Basic (High School), I = Intermediate (Undergrad), A = Advanced (Grad+).

Members posting new threads in the "physics" forums are asked to select a level. Staff may correct the level afterwards as they see fit.
Members can filter thread levels by using this tool. Also you may select the thread list prefixes as well.